1. Field of the Invention
This invention relates to a sliding valve which slides to its open position with minimal frictional resistance. The valve may be opened repeatedly without fluid flow therethrough causing wire drawing, flow cutting, or errosion of sealing components.
2. The Prior Art
The valve member of a poppet valve may be spring loaded. The spring force may be adjusted so that the valve member is movable to a position opening the poppet valve upon the application of any desired force, including a low force.
Valves having a sliding sleeve valve member presently do not have the responsiveness of a poppet valve. For example, the sleeve valve member generally carries two spaced seals. One of these seals is moved across the controlled flow port. However, when the valve member is in a position closing the flow port, both seals are subjected to a differential pressure. The pressure differential causes each seal to assume a position sealingly engaging an opposing surface. The sealing engagement of the seal generates a frictional force between the seals and the opposing surface. The frictional force retards movement of the sliding valve member. That frictional force can be reduced to approximately 40% of the pressure differential for each seal. Therefore a sliding valve having two seals requires a force of approximately 80% of the pressure differential to move the sliding valve member. For some applications, that required force is too large.
Some subsurface safety valves include a secondary valve. The secondary valve may be opened prior to movement of the primary valve towards its open position. Fluid pressures are thereby equalized across the primary valve prior to its movement towards its open position. The sealing surfaces for an equalizing valve may comprise metal-to-metal seats (see pages 3998-4002 of the "COMPOSITE CATALOG OF OILFIELD EQUIPMENT AND SERVICES" 1974-75 edition and U.S. Pat. Nos. 3,703,193 and 3,583,442) and/or a resilient seal element (see page 475 of the "COMPOSITE CATALOG OF OILFIELD EQUIPMENT AND SERVICES" 1976-77 edition). The flow area of the equalizing flow passage is relatively small. Because of the small flow area, a volume of fluid sufficient to feed a pressure generating pump cannot flow through the equalizing flow passage. However, enlarging the equalizing flow passage would increase the tendency of fluid flow therethrough to cause wire drawing of the sealing components. The wire drawing effect will increase if the equalizing valve is opened while a pressure differential exists. Once wire drawing occurs, flow cutting and errosion follow. Thereafter, the valve can no longer positively close the equalizing flow passage.